1. Field of the Invention
The present invention relates to a digital position control apparatus for controlling the position of an object to be controlled by digital information and, more particularly, to a digital servo control apparatus suitable for use in the position control of an optical head in a recording/reproducing apparatus of an optical disc or the like.
2. Related Background Art
In an optical disc, tracks are concentrically or spirally formed on the disc and are separated into a plurality of sectors in order to record variable length data or to make the accessing speed high and the optical recording/reproducing operation is executed on a sector unit basis.
To access an optical head to each sector and track, it is necessary to execute the position control (hereinafter, simply referred to as servo control) in what are called focusing and tracking directions of the optical head (mainly, the optical system). Such a servo control is largely classified into an analog servo control and a digital servo control in dependence on the difference of a signal form to be handled.
In recent years, the digital servo control is highlighted because of an advantage such that the setting of parameters can be changed from the outside.
FIG. 6 shows an example of a digital servo control apparatus which is used in a conventional optical disc recording/reproducing apparatus.
In the diagram, reference numeral 1 denotes an optical disc as a recording medium; 2 an optical system of an optical head; 3 a tracking error detector to detect a tracking error signal on the basis of an output from the optical system 2; and 4 a focus error detector to detect a focus error signal on the basis of the output from the optical system 2. A well-known detecting method in the optical disc recording apparatus or the like can be used for the tracking error detector 3 and focus error detector 4. For instance, a push-pull method is used to detect a tracking error and an astigmatism method is used to detect focus error, or the like. Explanation will be continued.
Reference numerals 5 and 6 denote A/D converters for A/D converting error signals which were output from the error detectors 3 and 4 into digital signals; 14 indicates a digital signal processing circuit (DSP); 10 and 11 D/A converters for converting the digital signals from the DSP 14 into the analog signals; 12 and 13 a tracking actuator and a focus actuator each for driving the optical system 2 of the optical head in a predetermined direction.
The operation of the optical disc servo control apparatus with the above construction will now be described.
First, when information is recorded or reproduced onto/from the optical disc 1 by the optical system 2, the position of the optical system 2 is controlled (hereinafter, simply referred to as a servo control) by the focus actuator 13 and tracking actuator 12. A tracking error signal and a focus error signal are detected from the reflected lights from the optical disc 1. The detected error signals are computed and processed by predetermined calculating equations by the digital signal processing circuit, thereby obtaining controlled variables. The actuators 12 and 13 are made operative on the basis of the controlled valuables. Information is recorded or reproduced to/from a desired track or sector by the above operations while executing the tracking and focusing operations.
However, in the above conventional example, each time the analog servo error signals are converted into the digital signals by the A/D converters 5 and 6, the controlled variables to the actuators are calculated by the DSP 14. Thus, there is a problem such that it always takes a predetermined time and the processing time corresponding to the computing processes is always included in the servo control system.